1. Field of Invention
The present invention relates to the field of producing membranes for chemical current sources, in particular, membranes with high oxygen permeability and at the same time with high barrier properties towards steam. Membranes of such type may be applied to dry metal-air current sources of various kinds: zinc-air, lithium-air, aluminum-air, etc.
2. Related Art
At the moment, in chemical current sources (CCS) of the above type, either calibrated holes for atmospheric air flow to air electrode, or separating materials based on polyvinylchloride or polyolefines similar to those used in accumulators are applied. Such separators are proposed, for instance, in Russian Patent No. 2017761 (porosity of 65.5-85.5% and pores size up to 1.03-8.6 .mu.m, and made of polyethylene with perchlorovinyl resin additive). However, though they, as a rule, have a sufficient oxygen permeability, they possess too high a steam permeability, thus they do not provide a steady operation of a metal-air CCS.
Hollow-fiber form membranes for air drying are known [U.S. Pat. No. 4,900,626]. They comprise a microporous membrane-support of polyvinylidene fluoride or other polymers, and coating based on bridged cross-linked polydimethylsiloxane which does not overlap the whole porous support structure, the surface density of coating on PVDF support being 9 g/m.sup.2. The porosity of membrane-support is 60%, size of the dense layer pores is 1-20 .mu.m, and that of loose layer is up to 100 .mu.m. Water vapor permeability of membranes with such porous structure is 260.times.10.sup.-5 cm.sup.3 /cm.sup.2 s/cmHg, Hg, that is more than one order higher than the required level, and oxygen permeability is 3.71.times.10.sup.-5 cm.sup.3 /cm.sup.2 s/cmHg, Hg, that is considerably lower than the required level.
The membranes nearest to the proposed invention are semipermeable asymmetric membranes of polyvinylidene fluoride [EP 0,330,072] which are characterized by an average porosity of more than 50% (preferably more than 80%), thickness from 50 up to 250 .mu.m (preferably from 70 up to 120 .mu.m), an average pore diameter from 0.01 .mu.m up to 1 .mu.m (preferably from 0.1 up to 0.3 .mu.m). However, their porosity and pores dimension are too high, and therefore the membranes do not possess barrier properties towards steam which are necessary for the effective operation of dry metal-air current sources. Thus, in Example 4 of the patent for the option of membrane on a fabric support the steam permeability value is 4400 g/m.sup.2 for 24 h, or about 18 mg/cm.sup.2 .multidot.h, that is two decimal orders higher than the maximum allowable steam permeability level.
Membranes for dry metal-air chemical current sources should meet specific requirements for permeability, in particular, a high permeability towards oxygen from atmosphere inside CCS in order to provide oxygen to the air CCS electrode, and at the same time a low permeability towards steam both from outside to inside, and from inside to outside, providing independence of CCS characteristics on the ambient atmosphere humidity, preventing electrode corrosion and keeping an optimum consistency of the paste electrode. The required characteristics of membrane permeability are defined by the following values:
Oxygen permeability--not less than 4.56 cm.sup.3 /min.multidot.cm.sup.2. (2,70 m.sup.3 /m.sup.2 .multidot.h) PA1 Steam permeability--not more than 0.178 mg/h.multidot.cm.sup.2.
Thus, the requirements for membranes for dry metal-air CCS are of specific nature and membranes produced for some other fields of application are not able to meet the said requirements.